Recently, for higher definition and lower power consumption of a liquid crystal display, a thin-film transistor (TFT) comprising a channel layer of an oxide semiconductor such as an oxide of an alloy of indium, gallium, and zinc (IGZO) is in actual use. In a coplanar TFT of this kind, it is necessary to laminate a gate insulator film and a gate electrode material on an island-shaped IGZO film. However, because of the influence of a step in an end portion of the island-shaped IGZO, it is difficult to form the gate insulator film and gate electrode material conformally in coating. Generally, a step even larger than the already existing step of the IGZO is created after the gate insulator film is formed, and therefore the gate electrode material deposited thereon can not be connected well in the end portion of the IGZO (step-disconnection).
In the case of a polysilicon thin film transistor, when the polysilicon film is processed by a dry etching method with an etching gas, the etching conditions are adjusted such that its end portion becomes oblique to some extent with respect to the substrate. In this way, the coating of the gate insulator film can be conformal. In the case of an IGZO film, however, since wet etching with oxalic acid or the like is mainly employed, only a substantially vertical end portion can be made. As a result, the step-disconnection of the gate electrode material occurs in the end portion. When the width of the channel is narrow, the step-disconnection results in disconnection of the gate portion of the transistor from the lead-out wiring portion for forming a circuit. This leads to a problem of incapability of transistor operation.
As a film formation which enables conformal coating, silicon dioxide film formation by the chemical vapor deposition (CVD) method using a source gas of tetraethyl orthosilicate (TEOS) is well known. This material is used as a gate insulator film disposed on a polycrystalline silicon film having large convexoconcave, and can moderate the convexoconcave of the polycrystalline silicone film thereunder.
However, the following was found. That is, when a silicon dioxide film was formed with gaseous TEOS on the IGZO film, oxygen was lost from the IGZO surface by a chemical reaction between the gaseous TEOS and the IGZO surface. As a result, the IGZO film, which was originally a semiconductor, behaved as if it was a metal at the interface with the gate insulator film and became unable to perform the transistor operation. This was similarly observed in the cases where metal oxides other than IGZO were used as the semiconductor layer.
As described above, conventionally, when a silicon dioxide gate insulator film is formed with gaseous TEOS on an oxide semiconductor, the oxide semiconductor behaves as if it is a metal at the interface with the gate insulator film, causing a problem of incapability of transistor operation.